1. Field of the Invention
The present invention relates to a joint structure for a robot, which includes a hollow-structured arm, a housing that forms support portions for swingably supporting the arm, and a motor that causes the arm to be swung.
2. Description of the Related Art
Known in the art is a joint structure for a robot in which a pair of support portions disposed in opposing relationship to each other swingably support an arm with the arm being held therebetween, or a so-called both-end support type joint structure.
Such a joint structure includes a hollow-structure arm, and a U-shaped housing that forms a pair of support portions disposed in opposing relationship to each other with one end of the arm held therebetween, and swingably supports the arm by the pair of support portions as disclosed in Japanese Patent Laid-open Publication No. H08-141968.
FIG. 3 is a sectional view of a prior-art joint structure for a robot. As illustrated in FIG. 3, a U-shaped housing 10 forms a pair of support portions A and B disposed in opposing relationship to each other with an arm 14 being held therebetween, and supports the arm 14 swingably about a swing axis X by the pair of support portions. In each inner space of the housing 10 and the arm 14 is accommodated a rotating device that causes the arm 14 to be swung. As the rotating device, use is made of a motor 15 and a speed reducer 17 such as illustrated in FIG. 3. As illustrated in FIG. 3, each axis of rotation of the motor 15 and the speed reducer 17 is located on an axis same as the swing axis X.
As illustrated in FIG. 3, the motor 15 is mounted to one wall portion 14a of opposite walls of the arm 14 in the inner space of the arm 14. Further, other wall portion 14b of the arm 14 is formed with an opening portion 14c, and a hollow bearing 16 is located in the opening portion 14c. A cylindrical hollow shaft portion 10a is inserted in the hollow bearing 16. The hollow shaft portion 10a is formed in the housing 10, and the center axis of the hollow shaft portion 10a is coincident with the swing axis X of the arm 14. Further, the inner diameter of the hollow shaft portion 10a is made to be larger than the outer diameter of the motor 15 such that the motor 15 is allowed to pass into the hollow shaft portion 10a, thereby making it possible to mount or exchange the motor 15. Thus, as the outer diameter of the motor 15 increases, it is necessary to increase the inner diameter D of the hollow portion of the hollow shaft portion 10a and the size of the hollow bearing 16 as well.
Generally, the cost of the hollow bearing increases as the bearing size increases. In the case of a wrist joint of a robot as disclosed in Japanese Patent Laid-open Publication No. Hei8-141968, the outer diameter of the motor to drive the joint axis becomes relatively small so that the size of the hollow bearing may be relatively small.
However, for a joint such as elbow joint or shoulder joint to which a larger load is applied than that applied to the wrist joint, it is necessary to use a motor larger than that for the wrist joint. Consequently, the size of the hollow bearing increases, and thus the cost of the hollow bearing also increases. Such a problem arises when the joint structure disclosed in Japanese Patent Laid-open Publication No. H08-141968 is applied to a joint other than the wrist joint of a robot.
FIG. 4 is a perspective view illustrating a vertically articulated robot. As illustrated in FIG. 4, a hand 41 for gripping a work W is mounted on a distal end of an arm of the vertical articulated robot. The joints of the arm may include a wrist joint 42, an elbow joint 43, and a shoulder joint 44. The motor to drive the joint axis of the elbow joint 43, the shoulder joint 44 or the like often becomes larger as compared with the motor to drive the joint axis of the wrist joint 42.
In order to avoid the above-mentioned problem of cost, a joint structure has been proposed in which the U-shaped housing 10 is formed of two components so as to be splittable. FIG. 5 is a sectional view of a joint structure including such splittable U-shaped housing 10. Meanwhile, in FIG. 5, components same as those illustrated in FIG. 3 are denoted by the same reference numerals.
Referring to FIG. 5, the U-shaped housing 10 is formed of a first L-shaped component 11 and a second I-shaped component 12 which are tightened together by bolts 13. The first component 11 and the second component 12 are disposed in opposing relationship to each other with the arm 14 being held therebetween, and support the arm 14 about the swing axis X. In particular, the second component 12 is formed with a cylindrical hollow shaft portions 12a. The hollow shaft portion 12a is inserted in the hollow bearing 16 existing in an opening portion 14c of other wall portion 14b of the arm 14. Further, the center axis of the hollow shaft portion 12a is coincident with the swing axis X of the arm 14.
With such structure, it is possible to remove the second component 12 from the first component 11 by loosening the bolts 13. Upon removal of the second component 12, the hollow bearing 16 can also be removed from the opening portion 14c of the arm 14. As such, it becomes possible to mount or exchange the motor 15 from the opening portion 14c merely by making the inner diameter of the opening portion 14c larger than the outer diameter of the motor 15.
In other words, with the prior art joint structure illustrated in FIG. 5, there is no need to make the inner diameter of the hollow bearing 16 and the hollow shaft portion 12a lager than the outer diameter of the motor 15. Thus, it is possible to determine the size of the hollow bearing 16 without depending on the outer diameter of the motor 15.
However, the outer shell of a robot is generally formed of a casting material. As such, the housing 10 is formed by combining the first component 11 and the second component 12, each of which is formed of a casting material, by the bolts 13. With such structure, the rigidity of the housing 10 is decreased as compared with a structure in which the housing 10 is entirely formed of a single cast component.
Further, since the housing 10 is formed by combining the first component 11 and the second component 12 with each other, a problem also arises that split line increases on the outer shell surface of the robot so that the appearance of the robot is deteriorated.
As above, in the joint structure for the robot disclosed in Japanese Patent Laid-open Publication No. H08-141968, there is a problem that the cost of the hollow bearing will increase as the outer diameter of the motor increases. In order to avoid this problem, there is a structure in which the U-shaped housing that supports the arm at both ends is formed of two components so as to be splittable as illustrated in FIG. 5. However, in this structure, there is a problem that the rigidity of the housing and the appearance of the robot are deteriorated.